Caption: Contrail. Contrail, or vapour trail, left by an aircraft flying at high altitude. This is an artificial cloud formed from frozen water droplets from the exhaust of the engines of an aircraft. It is formed when the aircraft is flying at an altitude exceeding 5000 metres and when the upper atmosphere is nearly saturated with moisture. In dry air the contrail quickly dissipates. Photographed over Yukon Territories, Canada.